Retroviruses can be used as true vectors for genes other than genes that lead to oncogenesis. A recombinant retrovirus containing the src gene of Harvey murine sarcoma virus (Ha-MuSV) and the thymidine kinase gene (TK) of Herpes simplex virus type 1 (HSV-1) were constructed and isolated. The new viruses can induce focus formation on NIH 3T3 cells and convert NIH 3T3 (TK-) cells into the TK+ phenotype by carrying into the TK- cells the HSV-1-tk gene. In the TK+ transformants, HSV-1-specific thymidine kinase can be identified by immunoassays. Hybridization analysis indicates that the recombinant virus contains both the Ha-MuSV src sequence and the tk gene sequence in a single RNA species of approximately 4.9 kilobases. The HSV-1 tk gene linked to the deleted Moloney murine leukemia virus (Mo-MuLV) genome in double recombinants. The mouse TK- cells can be transformed into the TK+ phenotype with much higher efficiency by transfection using recombinant DNA containing both the tk and the defective Mo-MuLV than using the tk DNA alone.